Umm......
I would probably put some super glue or the glue shoe repair professionals use where the tear is. After it dries, I would glue a small metal rod or something similar to that section making it stronger. The sheath will cover anything you do so no problems...
You could also try to get a very strong string and just sew the two parts together then put a small amount of glue on it to strengthen it...
Just a couple of ideas...